Thrust bearing devices of this kind, which are not only exposed to a considerable axial load, but in which only small oscillating rotational movements of one race relative to the other occur in a considerable part of the operational life, are used, for example, as blade suspension bearings for axial flow fans having blades which are adjustable during operation. Usually, the adjustable blades are retained in the wheel rim of the fan wheel by a suspension arrangement, in which a control arm connected with the blade foot and having also connection with a blade pitch adjusting mechanism, carries a flange which is connected with one race of the bearing, whereas the other race is secured relative to a surrounding bearing bore in the wheel rim.
During rotation of the fan, such blade suspension bearings are exposed to a very heavy axial pressure load from the centrifugal forces acting on the blade. Moreover, for a given blade pitch setting the rollers of the bearing will be in a nearly stationary condition relative to the races and only perform a small rocking or oscillating rolling movement around a position of equilibrium in synchronism with control pulses supplied to the blade adjusting mechanism. As a result of these two phenomena taken together, bearings of this kind are to a particular extent subjected to wear, which often results in deterioration of the races due to a phenomenon of surface impression fatigue, also known under the term "brinelling", whereby the rollers may be blocked relative to the races, so that the relative movability thereof is reduced or even completely prevented, and adjustment of the blade pitch is considerably restricted or completely blocked.
In spite of continuous efforts to remedy these problems for blade suspension bearings through improved bearing design and surface treatment of rollers and races, bearings of the kind referred to will, in practice, often suffer from the above mentioned disadvantages.
Similar problems occur in axial thrust bearings of the kind used in the front wheel suspension arrangement of vehicles, such as automobiles for transferring the weight of the vehicle to the rotatably suspended front wheels. Also in such bearings, a risk of brinelling occurs as a result of the fact that only part of the races of the bearing is utilized.
In this field, it has been suggested in the specification of U.S. Pat. No. 2,453,291 to remove the disadvantages mentioned by means of a thrust bearing device of the kind mentioned having two relatively movable coaxial roller races positioned in parallel planes and axially loaded relative to each other, said races constituting one of the races of each of two coaxial thrust bearings coupled in series with respect to their loading forces, in which bearings the two opposite races are immovable relative to each other and are prevented by a blocking device from rotating in one and the other direction of revolution, respectively, relative to one and the other of said relatively movable races.
In this prior art bearing design, the two series-arranged bearings will be alternatively active for one and the other direction of revolution. When one of the relatively movable races belonging to each of the two bearings is performing a rotational movement, whereas the other is retained against rotation, the opposite relatively immovable races belonging to each of the bearings will by movement in one direction of revolution take part in this rotational movement, whereas by movement in the other direction of revolution they will be retained against rotation. As a result thereof, each roller in each bearing will, in principle, by each rotational movement performed by the bearing in question be displaced to local engagement with another part of contact on the races in question.
Moreover, the design with two bearings coupled in series with respect to their loading forces results in a distribution of wear. Both the continuous displacement of the rollers relative to the races and this distribution of the wearing load contribute to a reduction of the risk of brinelling and thereby to an increase of the life time.
In the above mentioned prior art design, the blocking device comprises two ratched teeth mechanisms belonging to each of the bearings and each comprising two rims each connected with one of the race rings of the bearing, said rims being formed with ratched teeth facing and mutually engaging each other and designed such that the teeth are oppositely directed in the two ratched teeth mechanisms, which are coupled together by provision of the relatively immovable races on a common intermediate race ring for the two bearings.
In addition to the complicated construction, which makes the bearing device considerably more expensive, the above mentioned design of the blocking device in the known bearing construction will only be able to fulfil the object aimed at, i.e. secure a continuous displacement of the rollers relative to the races, for rotational movements beyond a certain magnitude corresponding to the tooth spacing of the ratched teeth rims. In view thereof and of the fact that in case of a great axial load, a considerable resistance must be overcome for each rotational movement due to the engagement of said ratched teeth, the prior art construction referred to will not be suitable for taking up small oscillating rotational movements of the kind occurring, inter alia, in blade suspension bearings as mentioned hereinbefore, which are exposed to a very heavy axial load from centrifugal forces. With the described design of the blocking device in the known bearing construction, such small movements will not give rise to any displacement of the rollers, because they are typically smaller than the minimum movement determined by the tooth spacing of the ratched teeth.